1. Field of the Invention
The present invention relates to a manufacturing process of strengthened and toughened bainitic nodular graphite cast iron (ductile cast iron) which is subjected to isothermal transformation treatment (hereinafter referred to as "austempering") to precipitate bainitic structure thereby obtaining a toughened material.
2. Prior Art
As is well known, a nodular graphite cast iron is generally obtained by the steps of adding explosively a small amount of Mg to a molten iron, spheroidizing the graphite morphology, and giving strength and toughness thereto. Since a series of successes in getting the speroidized graphite, studies and developments in the field have been directed toward the matrix looking for higher toughness thereof, and it has been proposed that the most useful method for the purpose is austempering, in other words, known as austempered ductile iron (ADI), which is now put in practical use in various components for machines.
It has been also found that because the nodular graphite cast iron thus austempered includes a large amount of Si which is a graphitization accelerating element, carbide which negatively affects the toughness is hard to precipitate, and a large amount of residual austenite is retained therein, which is very effective in improving mechanical properties of materials, enhancing thereby the application of such a treatment even more.
In the application of austempering, at first, full annealing of an as-cast product is carried out as a pretreatment, extending the results of studies about steel materials accumulated up to the present as well as principles based on such results. For example, as shown in FIG. 1 a-1, an as-cast product of nodular graphite cast iron is subject to two-stage annealing for full ferritization of the matrix thereof, and segregation of alloying elements, being contained in small quantities and existing microscopically in the as cost state, are completely diffused to obtain a homogeneous material. When necessary, a two-stage annealing, as shown in FIG. 11 a-2, is further applied to the ferritized material to be used as a prior structure, for full pearlitization of the base thereof, i.e., conversion to pearlitized nodular graphite cast iron.
The austempering is started either with such a ferritized matrix or pearlitized matrix as a prior structure, and after heating the material to reach its .gamma. range so as to fully austenitize the matrix and obtaining full solid solution of minor elements contained therein homogeneously into the austenite, the material is soaked into a predetermined isothermal salt bath and held therein up to the completion of transformation to bainite.
The foregoing is a conventional method for manufacturing strong and tough bainitic nodular graphite cast iron.
With the progress in obtaining tougher nodular graphite cast iron, the demand for this cast iron has been increasing particularly in such industrial fields as automobiles, machine tools and industrial machinery, and further tougher cast iron has been demanded. This is because it is quite attractive to get components for use in machines and equipment having sufficient toughness comparable to those of steel while maintaining its own properties of cast iron (such as abrasion resistance, heat resistance, corrosion resistance, lubricity, etc.).
Several attempts have been proposed in order to obtain further toughened bainitic nodular graphite cast iron each aiming at improvement of the matrix. In this connection, described hereunder is a summarized report on crack initiation and crack propagation which are confirmed by the inventor through fracture observation of a nodular graphite cast iron (the report titled "Fracture Toughness of Cast Iron" reported by Toshio Kobayashi and published in Bulletin of the Japan Institute of Metals 18 (1979) (512). Namely, in FIG. 12-A which shows the case of a ductile crack, large voids 2 (dimples) are formed under tensile stress (o) due to decohesion at a graphite-matrix interface 1 and with existing inclusions 3, small voids are formed and combined to form a crack. On the other hand, when a cleavage crack takes place directly in the low temperature range, it is often the case that the cleavage initiates at an eutectic cell boundary rather than at the graphite-matrix interface 1 itself. In this sense, FIG. 12-B shows a case in which: (1) dislocations are accumulated: (2) stress concentration occurs in the inclusions at the boundary of the eutectic cell 4 and in the carbide 5; (3) a cleavage crack 6 is thereby initiated. In the nodular graphite cast iron, it is often the case that a brittle crack is initiated at carbide or inclusion at the boundary between eutectic cells due to a segregation effect at the time of solidification.
Considering the foregoing observation, it is concluded that ductile cracks are generally initiated from the graphite portion and that the matrix structure has much effect in the event of cleavage fracture under low temperature. For that reason, removal of the brittle hard phase caused by segregation and full annealing or full normalizing aiming to refine ferrite grain size have been necessarily carried out as a pretreatment for austempering. However, there is a certain limit in the effect achieved by such pretreatment. Accordingly, a novel idea is needed to achieve a level of higher toughness.